Brown fat could help obesity, researchers say

When it's cold, huge numbers of calories burned

Fight fat with fat? The newest obesity theory suggests we may one day be able to do just that.

For more than 30 years, scientists have been intrigued by brown fat, a cell that acts like a furnace, consuming calories and generating heat. Rodents, unable to shiver to keep warm, use brown fat instead. So do human infants, who also are unable to shiver their muscles to stay warm. But it was generally believed that humans lose brown fat after infancy, no longer needing it once the shivering response kicks in.

That belief, three groups of researchers report, is wrong.

Their papers, appearing today in The New England Journal of Medicine, indicate that nearly every adult has little blobs of brown fat that can burn huge numbers of calories when activated by the cold, like sitting in a chilly room that is 61 to 66 degrees.

Thinner people appeared to have more brown fat than heavier people, younger people more than older people; people with lower glucose levels, presumably reflecting higher metabolic rates, had more than those whose metabolisms were more sluggish, and women had more than men. People taking beta blockers for high blood pressure or other medical indications had less active brown fat.

“The thing about brown fat is that it takes a very small amount to burn a lot of energy,” said Dr. Ronald Kahn, head of the section on obesity and hormone action at the Joslin Diabetes Center in Boston.

The hope is that scientists may find safe ways to turn people's brown fat on, allowing them to lose weight by burning more calories.

Finding a successful treatment for obesity would be a Holy Grail for scientists. Most obese and overweight people are unable to shed pounds and keep them off with dieting and exercise.

And despite plenty of effort, pharmaceutical companies have been unable to develop a medicine that helps people safely lose and keep off a significant amount of weight. Any drug that could do that would be a guaranteed blockbuster.

But researchers caution that while mice lose weight if they activate brown fat, it is not clear that people would shed pounds – they might unwittingly eat more, for example. The data on global patterns of obesity are not good enough to say whether living in a cold climate makes people thinner.

The best evidence for the effects of brown fat is from earlier studies in mice, said Leslie Kozak, a professor of molecular genetics at the Pennington Biomedical Research Center of Louisiana State University.

Recently, Kozak put mice predisposed to obesity in a cold room, 41 degrees, for a week. The animals activated their brown fat. As a result, they lost 14 percent of their weight, which constituted 47 percent of their body fat, while eating a high-fat diet with 2½ times more calories than they had consumed at a comfortable room temperature. “That's just by going out in the cold, without any drug treatment,” Kozak said. But, he cautioned, mice, small animals with a comparatively huge surface area, are easily chilled. “Put the mouse in the cold,” he added, “and it becomes a heat producing machine.”